Handling inconsistencies among Twisted releases

Using scrapy with Twisted 13 produce this exception if a user visit a not-defined resource. The patch should solve the issue.

```
	Traceback (most recent call last):
	  File "/Users/nopper/.pythonbrew/venvs/Python-2.7.3/frugal/lib/python2.7/site-packages/twisted/protocols/basic.py", line 571, in dataReceived
	    why = self.lineReceived(line)
	  File "/Users/nopper/.pythonbrew/venvs/Python-2.7.3/frugal/lib/python2.7/site-packages/twisted/web/http.py", line 1619, in lineReceived
	    self.allContentReceived()
	  File "/Users/nopper/.pythonbrew/venvs/Python-2.7.3/frugal/lib/python2.7/site-packages/twisted/web/http.py", line 1694, in allContentReceived
	    req.requestReceived(command, path, version)
	  File "/Users/nopper/.pythonbrew/venvs/Python-2.7.3/frugal/lib/python2.7/site-packages/twisted/web/http.py", line 790, in requestReceived
	    self.process()
	--- <exception caught here> ---
	  File "/Users/nopper/.pythonbrew/venvs/Python-2.7.3/frugal/lib/python2.7/site-packages/twisted/web/server.py", line 184, in process
	    resrc = self.site.getResourceFor(self)
	  File "/Users/nopper/.pythonbrew/venvs/Python-2.7.3/frugal/lib/python2.7/site-packages/twisted/web/server.py", line 701, in getResourceFor
	    return resource.getChildForRequest(self.resource, request)
	  File "/Users/nopper/.pythonbrew/venvs/Python-2.7.3/frugal/lib/python2.7/site-packages/twisted/web/resource.py", line 98, in getChildForRequest
	    resource = resource.getChildWithDefault(pathElement, request)
	  File "/Users/nopper/.pythonbrew/venvs/Python-2.7.3/frugal/lib/python2.7/site-packages/twisted/web/resource.py", line 201, in getChildWithDefault
	    return self.getChild(path, request)
	  File "/Users/nopper/.pythonbrew/venvs/Python-2.7.3/frugal/lib/python2.7/site-packages/scrapy/webservice.py", line 48, in getChild
	    return error.NoResource("No such child resource.")
	exceptions.AttributeError: 'module' object has no attribute 'NoResource'
```